1. Field of the Invention
The present invention relates to a technology for decoding a moving picture. More particularly, the present invention relates to a method for error concealment in the decoding of a moving picture, and a decoding apparatus that conceals errors in accordance with the method.
2. Description of the Related Art
Moving pictures are transmitted over a number of different systems whereby it is desirable to use a coded transmission that often includes compression of the images. Most moving picture coding standards, which include H.263, H.264 and MPEG-4, employ relatively efficient image compression techniques such as variable length coding (VLC), spatial and motion prediction, etc., in order to reduce the bandwidth necessary to transmit and receive moving pictures, and reduce the transmission time as well.
However, variable length coding has problems in that it is very vulnerable to an erroneous channel environment. For example, even a few bit errors may cause a loss of synchronism during image decoding. Furthermore, variable length coding utilizes a prediction coding technique that can lead to error propagation throughout the whole image. Accordingly, the use of variable length coding can often lead to the quality of a restored image being significantly poorer than the original due to the susceptibility to errors occurring during channel transmission, as well as storage medium recording and/or reading.
In order to effectively address the aforementioned transmission errors, which are inevitable with present transmission technology, research has been conducted on coding techniques robust to errors and error concealment (EC) techniques that minimize damage due to errors occurring in a transmission environment.
There are a variety of other EC techniques available, including a post-processing technique, which is in widespread use. The post-processing technique assigns responsibility for EC to the decoder. The post-processing technique is an EC technique that can be somewhat effective for recovering erroneous information by using such a characteristic that, in the case of a natural image, provides for spatially or temporally adjacent pixels to change smoothly. According to the post-processing technique, an erroneous block is recovered in units of pixels or macro blocks by using a pixel value or motion vector (MV) of a block neighboring the erroneous block.
Accordingly, the post-processing EC technique is largely classified as a temporal EC (TEC) scheme and a spatial EC (SEC) scheme. In the TEC scheme, through the use of a high similarity between temporally successive images, a block most similar to an erroneous block in a present image is found in a reference image to the present image, and the erroneous block is concealed by the block most similar found in the reference image. In the SEC scheme, assuming that pixels neighboring in a spatial domain show no abrupt change in brightness, an erroneous portion is concealed by using pixel values of normally decoded blocks around the damage portion.
In general, the SEC scheme is favored for the error concealment of a still image or an image coded in an intra mode, and the TEC scheme is favored for the error concealment of an inter frame. Most moving pictures are coded using an inter frame in order to achieve high coding efficiency, and thus the TEC scheme exhibits higher recovery efficiency. The most popular TEC technique is a decoder motion vector estimation (DMVE) technique for finding a motion vector so as to recover an erroneous block in a decoder.
The DMVE technique is a technique on the decoder side that is similar to a motion estimation process in an encoder, which is a process of finding a block exhibiting a characteristic similar to that of a specific block by comparing the specific block with a reference frame. The DMVE technique also utilizes error concealment wherein a macro block having the highest similarity to a block requiring error concealment is found in a reference fame, and the corresponding macro block is recovered using the found similar block.
FIG. 1 illustrates the concept of a DMVE technique for error concealment in decoding of a moving picture that is sometimes used. Referring to FIG. 1, in order to conceal an erroneous macro block 102 in a present frame, a search window 110, which is appropriately set larger than the corresponding erroneous block while containing the same, is established in a previous frame, and respective blocks are searched in the corresponding search window 110. Error concealment is conducted by finding a macro block 112 having adjacent pixels (for example, pixels within appropriate upper, lower and left ranges) with the smallest differences from those of the erroneous macro block 102, and then the erroneous macro block 102 in the present frame is thereby replaced by the found macro block 112. The reason that the macro blocks appear to have only three dark sides is due to the characteristic of the H.264 codec, wherein it is not feasible to use information about pixels on the right side of a macro block because these pixels are always erroneous. In some cases it is also impossible to use information about pixels positioned on the lower side of a macro block. Further, respective pixel values are compared by using a scheme in which a Sum of Absolute Differences (SAD) is ascertained. An example of the aforementioned DMVE technique is disclosed in a paper “A cell-loss concealment technique for MPEG-2 codec video”, IEEE Transaction on Circuits and Systems for Video Technology (published in June, 2000), which is hereby incorporated by reference in its entirety as background material.
Although the exemplary DMVE technique described above provides comparatively high recovery efficiency, a more accurate error concealment technique is needed. The need in the art is apparent as soon as the corrected image is compared with an original image. In view of the quality issues with DMVE, the DMVE technique requires a large amount of calculations in order to be effective. In addition, DMVE, as well as the other EC techniques discussed above, has a problem in that it is not easy to apply to a multi-hypothesis technique in which high recovery efficiency can be achieved by a combination of two or more error concealment techniques.
The multi-hypothesis technique is a technique for conducting error concealment having higher stability, whereby two or more previous reference blocks are combined so as to recover an erroneous block, and can provide comparatively accurate error concealment because of the utilization of two or more independent methods. However, the multi-hypothesis technique has a problem in that the amount of calculations increases even more than the use of most of the single techniques. An example of such a multi-hypothesis technique is disclosed in a paper “Multi-hypothesis error concealment algorithm for H.26L video”, Journal of ICIP2003 (published in September, 2003).